The field of the invention relates to electrical conductors and, more specifically, electrical conductors in power generation systems subject to stress forces.
Conventional power generators generate electrical energy by means of induction. Such generators employ a stator core along with a rotor shaft having rotor coils associated therewith to rotate within the stator core in order to convert mechanical energy into electrical energy. Within the rotor shaft, extending axially relative to the lengthwise extent of the rotor, a pair of axial leads carry electrical current. To electrically connect an axial lead to the rotor coils associated with the rotor shaft, a radial stud often extends from the surface of the rotor shaft into the shaft to connect to the axial lead. A heavy conductor, usually positioned within a slot formed in the rotor shaft, electrically connects the axial lead to the rotor coils (usually the inner coil, which itself is connected electrically to the other coils).
These heavy conductors are often referred to as xe2x80x9cJ-leadsxe2x80x9d because their shape resembles that of a xe2x80x9cJxe2x80x9d lying on its side. As illustrated in FIG. 1, the conventional J-lead 12 comprises a straight axial portion 14 connected at one end to the radial stud 32xe2x80x2 and extending in an axial direction relative to the rotor shaft 24xe2x80x2, and, at the opposite end of the straight portion, a radial portion 16 that extends outward in a radial direction to connect to the rotor coil 28xe2x80x2.
The J-lead conductor 12 often is subject to stress forces stemming from several distinct sources. For example, as the rotor shaft spins within the stator core, centrifugal forces are generated. The rotor coils are positioned within axial slots 34xe2x80x2 extending along the length of the rotor shaft 24xe2x80x2 and retained therein by a retaining ring 30xe2x80x2 surrounding a portion of the rotor shaft 24xe2x80x2. Centrifugal force deflects the retaining ring 30xe2x80x2 and tends to pull the rotor coils away from the rotor shaft 24xe2x80x2 as it rotates at an extremely rapid speed. The J-lead 12 connected to the inner rotor coil 28xe2x80x2 correspondingly experiences a stress force acting in a radial direction relative to the rotor shaft 24xe2x80x2 and tending to pull the J-lead 12 radially away from the rotor shaft 24xe2x80x2.
The J-lead also experiences stress forces due to thermal expansions acting in an axial direction relative to the lengthwise extent of the rotor shaft. Because the J-lead and the rotor coils are conductors, electrons move at constant average xe2x80x9cdriftxe2x80x9d velocity through the lead and coils experiencing continual collisions with the atoms of the crystalline structures of the lead and coils. These collisions, of course, generate heat and cause thermal expansions of the J-lead 12 and the rotor coils. Because one end of the axial portion 14 of the J-lead is fixedly connected to the radial stud, thermal expansion of the J-lead often is axially biased, causing the end opposite to the one connected to the radial stud 32xe2x80x2 to move farther away from the radial stud 32xe2x80x2. At the same time, thermal expansion of the rotor coil 28xe2x80x2 generates a stress force in the opposite direction acting on dint portion 16 of the J-lead that extends outwardly in a radial direction from the rotor shaft 24xe2x80x2 and attaches to the coil. As the coil expands, the portion of the J-lead that is attached to the coil is pushed toward the radial stud 32xe2x80x2.
Thus, the net effect of these thermal expansions is that as one part of the J-lead 12 is forced away from the radial stud 32, another part is forced toward the radial stud 32xe2x80x2. The first force is due to a force acting on a first portion of the J-lead 12 positioned close to the rotor shaft, the second force is on a second portion of the J-lead 12 above the first portion and connected to the rotor coil. The result is that medial portions between the first and second portions of the J-lead are subjected to forces causing internal deformation.
In addition to the centrifugal forces due to rotation of the rotor shaft 24xe2x80x2 and the deforming forces due to thermal expansion, the J-lead 12 is also subjected to a variety of other forces including those stemming from vibrations within the generator and accentuated by possible pre-stress owing to the manner of installation and manufacturing variations.
In an attempt to accommodate these various forces, J-leads have conventionally been designed and manufactured to purposely permit internal deformation. Some J-leads, for example, are formed by joining in parallel several, reduced-diameter conductors or laminations. A persistent problem, however, is how to find a material that both permits the lead to flex and bend in response to stress forces and also acts as a good conductor. Copper, for example, is a good conductor, but has very poor fatigue properties. In addition, copper is difficult to form multiple parallel conductors into a J-lead and simultaneously provide adequate mechanical support. Components that provide mechanical support and bending capability require tightly controlled tolerances in manufacturing. The parts forming the J-lead must be very carefully assembled in accordance with very complicated procedures.
Not only are these procedures costly, they are fallible no matter how rigorously performed. Firstly, it is extremely difficult to calculate all initial pre-stress factors and J-lead stress forces under different operating conditions. Secondly, it is very difficult to predict how dimensional variations and other factors will influence the stress forces on the J-lead. Indeed, despite complicated attempts to engage in finite element analysis and conduct rigorous fatigue simulations, there continue to be notable J-lead failures.
In view of the foregoing, the present invention advantageously provides a conductor that substantially avoids internal deformation due to various stress forces on the conductor while providing a conductive path between distinct components of a generator. The substantial avoidance of internal deformation in the conductor makes the failure of the conductor considerably less likely. Specifically, reducing or eliminating entirely internal deformation accordingly reduces the probability of cracks in the conductor structure arising from stress-induced structural fatigue. Hence, a sectioned conductor of the present invention is much more reliable than devices like the conventional J-lead since the internal deformations that can cause fatigue leading to a breakdown are substantially eliminated.
Another distinct advantage of the present invention is the ease and efficiency with which a sectioned conductor according to the present inventor can be manufactured and installed. Instead of requiring intricate manufacturing steps to align and connect multiple, small-diameter conductors into a monolithic J-lead, separate pieces of the sectioned conductor are electrically joined easily and efficiently.
According to the present invention, a sectioned conductor is formed to have at least two sectioned members that remain electrically connected while being able to move relative to each other in response to stress forces. In the context of a power generator, the sectioned conductor includes at least a first sectioned member connected to a radial stud and a second sectioned member connected to at least one rotor coil to thereby provide a conductive path between the radial stud and the rotor coil. In a preferred embodiment, the second sectioned member of the conductor is adapted to respond to centrifugal forced generated by the rotation of the rotor within a stator by moving radially and independently of the remainder of the conductor as the coils and the retaining ring to which they attach move radially away from the rotor in response to the centrifugal force. Yet, as already noted sectioned members remain electrically connected even as the second sectioned member moves relative to the first.
The second sectioned member, moreover, is adapted to move axially in response to thermal expansion of the rotor coil resulting from current-induced temperature rises in the rotor coil. The second sectioned member is also free to move in substantially any direction in response to a combination of axially and radially directed forces such as those stemming from vibratory motions in the rotor.
As explained more fully herein, the present invention also encompasses related methods for reducing or eliminating stress forces in an electrical conductor. More specifically, the present invention provides a method for accommodating stress forces on an electrical connection while providing a conductive path between at least two spaced-apart electrical components within a generator. The method includes positioning a first portion of a conductor so as to electrically connect the conductor to a first component of at least two electrical components, and positioning a second portion of the conductor to electrically connect to a second component. The first and second portions of the conductor, moreover, are adapted so as to permit the second portion to move relative to the first portion in response to stress forces while remaining electrically connected to the first portion. The second portion, more specifically, moves relative to the first so as to substantially avoid internal deformation of the conductor Thus, among the other advantages, the present invention provides a method for providing and maintaining a conductive path with an electrical conductor that is substantially free of stress-induced internal deformation.